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Sunday, 9 October 2016

It is something of a trope that prehistoric animals must bare their teeth in palaeoart, even when their mouths are closed. Historically, the majority of palaeoartists covered the teeth of their subjects with lips, cheeks or other types of tissues and only select species – sabre-toothed carnivorans or mammoths – were depicted with exposed tusks or sabre-canines. This changed when artists working in the 1980s and 1990s - Paul, Hallett, Stout - and a certain 1993 movie started showing predatory dinosaurs with toothy overbites and perpetually exposed teeth. This convention has since expanded to all kinds of prehistoric animals, and some galleries of Deep Time now have more toothy grins than a holiday photo album. Theropod dinosaurs in particular are almost always shown with alligator-like overbites that perpetually expose their upper teeth, the large canines of stem-mammals protrude over their lower jaws, and even herbivorous animals with relatively unimpressive dentition (like sauropods) are shown without lips or other forms of dental covering.

Many words – mostly published at blogs, online mailing lists and social media - have been typed to discuss the credibility of lipless palaeoart, but the subject has traditionally received only cursory attention from academics. Happily for artists, this is starting to change. A small set of literature exists which debates the presence of extra-oral tissues in dinosaurs (e.g. Ford 1997; Knoll 2008; Morhardt 2009; Keilor 2013; Reisz and Larson 2016), and most of this agrees that some sort of soft-tissue - at least 'lips' - covered their teeth. However, a running theme of these works is that reliably inferring soft-tissues of the face is not a simple task, and we really need more data to be sure of anything. Work on more recent fossil mammals shows more reliable inferences (e.g. Wall 1980; Antón et al. 1998), obviously benefiting from soft-tissue data from a range of extant, close relatives. New insights on the evolution of mammal cranial nerves are helping to understand the development of sensitive lips and cheeks in stem-mammals (Benoit et al. 2016). It's still early days for understanding fossil facial tissues, but at least it feels like we're off the line.

Collectively, there seems to be recognition among the academics interested in this topic that understanding the tooth coverage of fossil animals lies largely in understanding living animals. Attempts to understand tooth exposure from skulls alone - through making inferences about tooth size, jaw closure and speculations on how extensive soft-tissues can be before they become untenable - do not consider all necessary data. For example, Prehistoric Times palaeoart adviser Tracy Ford (1997) looked solely at the skulls of predatory dinosaurs to infer the absence of lips, suggesting their teeth were so long that they would pierce lip sheathing once the jaws were closed. This study assumed that predatory dinosaurs closed their mouths to the extent that the teeth of the lower jaw contacted the roof of the mouth, and that the preserved tooth configuration was the condition in life. These points are common issues raised against lipped dinosaurs, but there are several major problems. Dissections and CT scans of reptile heads show that jaw muscles and other soft-tissues have a major influence on mouth closure, to the extent that reptile jaw skeletons are typically loosely closed under their skin, even when the mouth is fully sealed. Taphonomic studies show that teeth slip readily from their sockets after death and often fossilise in far more vampiric states than they were in life. And undermining this further is that no extant taxa with lipped jaws were used to calibrate a limit for oral soft-tissues. Arguments about tooth coverage based on simply looking at skulls, without detailed consideration of modern animals and their anatomy, border on being arguments from incredulity: "I don't believe the anatomy could do that."

Modern animals and their tooth coverage

For an upcoming project, I've been trying to crystallise my approach to restoring ancient animal facial tissues, and deciding whether to cover their teeth or not is an important part of that discussion. I've been deliberately broad in this assessment to attempt to try to sort the wood from the trees: discussions of oral tissues can sometimes get lost in the minutiae of tissue types, uncertain osteological correlates and so on - and many of these discussions result in the same answer: they can't be resolved with current data. That's not to say they aren't important discussions, but it's helpful to step back to see if we can answer the simpler questions as well: what gauge of teeth can be covered by oral tissues? When are teeth actually exposed? And what questions should we, as palaeoartists, be looking to answer when restoring facial tissues?

Reviewing literature and galleries of modern animals, we can see that overwhelming majority of living tetrapods have covered teeth, including all amphibians, most mammals and most reptiles (excluding birds, naturally. Hey, if they wanted to be involved in this post they shouldn't have lost their teeth). Exposed teeth are actually really rare, and a character completely absent in many major clades. The soft-tissues involved in covering the teeth are variable, but 'lips' – either slightly fleshy margins of skin, or skin overlying muscle - are so universal among tetrapods, as well as living relatives like lungfish, that we might assume lip tissues of some kind were ancestral to the group, and breaching these with large teeth is a derived condition evolved independently in a minority of lineages. Crocodylians are the only living tetrapods with fully exposed teeth, but it's increasingly obvious that they're also pretty specialised/derived/downright weird (Grigg and Kirshner 2015). Far from being 'living fossils' frozen in evolution, they have so many anatomical nuances and specialisations that their use as model organisms for other extinct taxa is increasingly questionable. This applies to aspects of their facial anatomy too - we’ll discuss this in more detail below.

Looking inside animal heads (above) shows that facial soft-tissues can cover very, very large teeth – perhaps much larger than we might intuitively expect. Examples from a range of tetrapods – including rhinoceroses, sloths, tapirs, mandrills, baboons, camels, tuataras, snakes, peccaries, bullfrogs, hippopotamuses, monitor lizards, clouded leopards, numerous rodents and others – show that large fangs, robust tusks and other forms of enormous dentition can be retained within lips or cheeks. These large teeth are truly ‘hidden’ without bulges, changes in lip direction or other features to betray their presence, and are thus undetectable unless their owners open their mouths (and sometimes not even then). Many people are shocked by the size of animal teeth when they see their skulls, and the savagery of mammalian herbivore dentition – horses and camel fangs, rhino tusks, baboon canines - are particularly startling.

We owe many of these surprises to animal lips, which are generally much more extensive than we casually assume. Large teeth can slide into soft-tissue sheaths located between gums and lips, and these are quite visible in the open mouths of some species. Amphibians, lizards and many mammals have upper and lower lips of similar size which meet over the teeth and sheaths can form on either jaw, but some mammals – including most carnivorous forms - have very large, fleshy upper lips over thinner, tightly-bound soft-tissues of the lower jaw (Antón et al. 1998). In these species, the canine teeth overbite the lower lip but the upper ‘over-lip’ is large enough to obscure the fact that the tooth is outside the lower mouth tissues. I am unaware of a reversed situation with the lower lip covering a thin upper lip: this may reflect the fact that overbiting dentition is much more common than underbiting. Regardless of the specific configuration, it is clear that we should not underestimate the capacity for facial tissues to obscure even very large, sharp and ferocious-looking teeth. The assumption that all conspicuous teeth of fossil animals were on display in life is thus problematic and does not agree with what we can observe in modern animals (below).

Applying palaeoart-esque considerations of oral tissue capacity to modern mammals suggest hippos are giant hogbeasts and mandrills evolved in Mordor. Restoring modern animals using palaeoart approaches is a completely original concept which in no way owes anything to some book called All Yesterdays (Conway et al. 2013).

When do teeth breach the confines of soft-tissue? Mostly, it seems teeth used to process food remain covered. Mammal tusks and the exposed canines of certain deer are not directly involved in food processing, although this is not to say they are non-functional overall (e.g. elephants use their tusks to break branches, dig, topple trees; deer fight with their large canines). It seems that teeth of extreme size relative to the rest of the dentition are most likely to escape covering with soft-tissue, and it helps – though is not mandatory – if they grow obliquely or directly away from the jawline (this accounts for the majority of living mammal tusks). Teeth can remain covered even when their tips extend to the dorsal or ventral limits of the jaw skeleton, so long as they are aligned more or less vertically within the jaw (e.g. the mandrill skull illustrated above).

What's up with crocodylians?

The elephants – or rather large semi-aquatic reptiles – in the room here are crocodylians: why do they have exposed teeth when all other tetrapods have largely covered mouths? Their teeth are not overly large, nor acutely angled. Some (Reisz and Larson 2016) have argued crocodylian dentition is only possible because of their semi-aquatic habits. The (unpublished, currently conference abstract only) Reisz and Larson hypothesis is that exposed teeth – specifically their enamel component – are at risk from desiccation and breakage without constant hydration from saliva or environmental water (Reisz et al. 2016). This is an interesting idea which potentially gives artists a useful, practical guide to restoring prehistoric animals: anything living outside water with enamel-covered teeth must have covered them with soft-tissue. Despite its unpublished status, this idea has already chimed with some quarters of the online palaeoart community who're restoring anything with enamel-covered teeth with full sheathing.

We need to talk about enamel and exposed teeth. The exposed canines of male wild boars, Sus, have enamel (white shading) coatings on 3/4 of their surface, despite being exposed (dentine is dark grey, cementum is light grey). What does this mean for the enamel desiccation hypothesis outlined above? Image from Hillson (2005).

However, this proposal may not be as simple to implement as it first appears. For one thing, there is a real lack of consistency in tusk composition in living animals (see Hilson 2005). It is true that, as noted by Reisz et al. (2016), the tusks of elephants have caps of enamel and cementum that wear off rapidly, leaving their tusks composed of dentine alone. This would seem to support the desiccation hypothesis, it implying that enamel is a liability outside of the jaw soft-tissues. However, living elephants may be atypical in lacking enamel on their tusks, there being fossil and living mammals which do have substantial enamel components on their exposed teeth. For example, the tusks of several gomphothere species have broad bands of enamel along their lateral surfaces, even as adults (Padro and Alberdi 2008), while the canines of male musk deer are enamel covered on the external surface. The tusks of male wild boars and warthogs only bear dentine on the posterior surface and wear facet, the rest of these large, exposed teeth being covered in enamel. The enamel components of these tusks are not just small caps that get worn off, but expansive coatings that persist on the tooth indefinitely and influence tooth wear (Koenigswald 2011). To confuse things further, walruses have dentine tusks like elephants, despite their aquatic habits seemingly precluding desiccation as a risk for their teeth, and the spiralling tusks of another marine mammal, the narwharls, are covered in enamel. If there is a relationship between enamel and tooth exposure, it is clearly a complicated one, and the presence of absence of enamel in itself seems to have little bearing on this topic in at least modern mammals. (Readers interested in tooth composition should check out the second edition of Samuel Hillson's Teeth (2005), for its extensive documentation and illustration of mammalian dentition).

Musk deer, Moschus, canines in lateral and medial view. Note the (white) enamel layer on the lateral surface, but dentine (grey) on the medial. From Hillson (2005 - the scale bar is likely erroneous!).

Our second reason to be sceptical of the enamel desiccation hypothesis concerns crocodylian behaviour. It is not widely appreciated that several crocodylians species ‘hibernate’, or more accurately aestivate, for months at a time in dry underground burrows during the hottest summer months (Grigg and Kirshner 2015). During these intervals they do not access water at all. Other, South American species spend dry spells as fully terrestrial carnivores, abandoning aquatic habits and obtaining water largely from the prey they kill (Grigg and Kirshner 2015). These states have to be explained against the suggested need to frequently moisten crocodylian teeth, because they suggest dental desiccation is not as risky as we're all assuming it is. Alternatively, it suggests that the requirement for hydration is so relaxed – literally months can pass without getting the teeth wet – that it probably has little influence on tooth anatomy.

Furthermore, there are important caveats about crocodylian facial tissues that we have to factor into any discussion of their lipless configuration. Crocodylian faces are far more specialised and unusual than they first appear, and this may factor into their lipless mouths. Their highly keratinous facial skin undergoes a developmental pathway unlike that of any other amniote (their facial skin is essentially one, highly 'cracked' scale) (Milinkovitch et al. 2013) and their heads are riddled with hyper-sensitive Integumentary Sense Organs (ISOs). ISOs are a unique crocodylian feature and are attuned, among other things, to sensing tiny vibrations in water (Soares 2002, see Grigg and Kirshner 2015 for a recent overview). In at least some parts of the crocodylian skull ISOs are situated over tiny foramina, presumably housing nervous tissues, and the overlying epidermis is thinned, with a reduced keratin component, to enhance their sensitivity (Soares 2002). We can thus see that ISOs do have a role to play in configuring crocodylian skin, and they present many questions that palaeoartists should be interested in. Are ISOs an important reason for crocodylian faces having such tight, contour hugging and lipless skin? Do the major functional and developmental distinctions of croc faces explain the lack of crocodylian lips? It might explain why virtually no other aquatic tetrapods have abandoned lips - aside from the the odd (and perhaps only) exception like the South Asian river dolphin*, there are no whales, snakes, seals or otters with crocodylian-like, fully exposed teeth. And given that no other lineages have osteological correlates for ISOs, should we put huge caveats around using crocodylians as models for facial tissues in anything other than their own ancestors? I don't know if anyone has answers to these questions yet, but they're food for thought when using crocodylians as ammunition for lipless reconstructions of fossil animals.*Thanks to Ádám Lakatos for pointing out the toothiness of some river dolphins!

It's still very early days for the enamel/oral covering hypothesis, but modern animals suggest that interpretations of enamel precluding extraoral teeth are definitely more complicated than they first appear, and may even be flawed. If so, the simple presence of enamel on the teeth of fossil organisms may not be as useful to artists as some are currently suggesting. But this conclusion is preliminary, and we need to wait for this idea to mature before it's shot down entirely. We know, for instance, that there's more than one type of enamel among vertebrates. Reptilian enamel, for instance, is both thinner and of different microstructure to mammalian enamel, and these clades have rather different approaches to tooth longevity. This may mean something for enamel desiccation and long-term tooth exposure, and we may think differently on this matter once this research has been completed.

Predicting tooth exposure in fossil species

Fully 'lipped' gorgonopsids and theropods: maybe not be as exciting as their toothy variants, but are they more credible? Well... if modern animals are anything to go by, probably.

All this said, what can we say about the decisions to show prehistoric animals with exposed teeth? My reading of modern tetrapods is that covered teeth is their ‘default’ configuration, and we should apply the same logic to extinct animals. If so, maybe only the more extreme examples of fossil dentition should qualify for perpetual display. Perhaps instead of asking ‘does this animal have lips?’ we should ask why they should not have them. We have to concede that the dentitions of many fossil animals frequently shown with exposed teeth – particularly theropod dinosaurs, gorgonopsids and other carnivorous stem-mammals – are relatively no larger, and in some cases a great deal smaller, than those enclosed inside the oral tissues of living animals, especially once taphonomic tooth slippage is corrected (above). For these species, it is very difficult to justify why their teeth should not be covered.

If this is so, only especially long teeth which project a considerable distance from the margins of the skull and lower jaw should be considered strong candidates for permanent exposure. Select examples might include the canines of certain mammalian carnivores (e.g. Smilodon and other machairodont felids), the tusks of fossil elephants and their relatives, and the larger tusks of dicynodonts. We should also note those fossil reptiles – such as certain crocodyliformes, pterosaurs and marine reptiles – where entire toothrows are composed of dentition so long that their tips extend well beyond the margins of the jaw skeleton. Such extensive dental apparatus would seem to preclude the development of any sheathing tissues, at least akin to those exhibited by from modern animals, and these animals probably had fully exposed toothrows in life. Of course, this conflicts with the observation that food-processing teeth are almost always covered in the modern day. However, we can defend this approach by arguing that their morphology gives a strong reason for ignoring this guideline: it answers the "why we shouldn't give them lips?" question.

The large, procumbent dentition of plesiosaurs and certain pterosaurs argues against them being sheathed in life, although I do wonder if some plesiosaurs are in a 'grey area' here. Could animals like Leptocleidus (right) have covered its teeth with lip-like tissues? Hmm....

We might also set aside this guideline when extant relatives of modern forms provide us with means to predict unusual lip anatomy. For instance, the aforementioned ‘over-lip’ of modern mammalian carnivores is common enough across this group to assume it was present in their fossil relatives. Because we understand how the lips of these animals work, we can make more specific predictions concerning tooth exposure in species with particularly impressive teeth. Thus, we can look at classic reconstructions of machairodontid cats like Smilodon with perpetually bared fangs as reasonable because, unless their lips were arranged differently to virtually all their living relatives, that’s simply how their lip tissues would respond to a massive set of canines. And yes, I'm aware of Dunae Nash's recent discussions about sheathing Smilodon: given that this rests heavily on enamel being a no-no in exposed teeth, I'm unconvinced for the reasons explored above.

The concluding caveat

Of course, it must be reinforced that these are just guidelines - and guidelines based on my own qualitative studies, nonetheless, your mileage with them may vary - and there are exceptions to the suggestions made above. As is well known, for all the suggestion that restoring sabre-toothed cats with exposed teeth is reasonable, one living cat species – the clouded leopard – does cover a set of long upper canines in a lower lip sheath. We would not predict this based on other cat species and, if known only from fossils, clouded leopards would probably be restored with slightly exposed canines. Likewise, the exposed tusks of some deer are not especially massive, and if we followed the suggestions above we'd probably cover them up in a reconstruction. But palaeoart is ultimately a game of prediction and probability, attempting to restore what is most likely to fill gaps in our data, and any game of odds will have some failures. That’s not to say we shouldn’t ignore these exceptional examples - they show that guidelines can't be trusted all the time - but it makes sense for us to know where the guidelines are in the first place. As with all aspects of palaeorestoration, all of us stand a chance to be proved wrong about our artistic decisions: if and when that happens, the best we can hope for is to have been wrong for the right reasons.

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61 comments:

The presence of absence of lips in non-mammalian synapsids has always been very fascinating to me. The fact that dicynodonts managed to produce keratinous beaks - a no-no in the muscular lipped true mammals - only adds fuel to the fire.

I know of the hairy coprolite, yes, but I deliberately didn't fuzz the gorgonopsid because the group lacks the shortened cranial nerves recently linked to sensitive facial hair (whiskers etc.) and in many respects they aren't that mammal-like - sprawling forelimbs, semi-sprawling hindlimbs, only just starting to developing mammal-like jaws, etc. Not that hair has to coincide with mammal-like osteology of course, but I took these as an indication of general mammaliness. I was going for 'stem-mammals do monitors': creatures which are on the cusp of being more active and energetic than more rootward amniotes, but not quite there yet.

To an extent, yes, but gorgonopsids still have relatively primitive limb girdles and limb mechanics. Other stem-mammal sprawlers (or semi-sprawlers) show further movements towards the mammalian condition. Again, this might have nothing to do with hair etc., but in terms of general proximity to the crown mammal condition, gorgonopsids seem pretty far off.

"In regards to the jaws, where does this new study on synapsid face nerve place them?"

Full details (should be accessible): https://www.researchgate.net/publication/302413151_Palaeoneurological_clues_to_the_evolution_of_defining_mammalian_soft_tissue_traits

A very well-written post! I have been looking forward to seeing gorgonopsids reconstructed with lips for some time, never getting to illustrate any myself, and you have really done such a fascinating group justice! And while the all the figures are stunning, my favourite in this post is the Tyrannosaur (is that Magneto or Judge Dredd?).

I do hope that Reisz and Larson will adapt their theory based on the evidence you listed above. After all, it is the nature of a good theory to continuously evolve based on new evidence. The presence of lips in theropods also helps explain the small conundrum of drinking, as theropods with lips would have definitely been able to hold water in their mouths much better than had they not possessed lips.

One group that you did not mention was the Mosasaurs. They are in almost all analyses placed with Iguanians, Snakes and Anguimorphs, (as well as Polyglyphanodontians), and as such should have similar oral anatomy (particularly lips and tooth-covering gums) to modern members of these groups, especially the aquatic members. I explain the reasoning here (https://bonesharpesite.wordpress.com/2016/10/02/gummy-mosasaurs/), if you would like to take a look.

While Smilodon probably had its teeth exposed, I still think it's likely other sabertooths like Homotherium had their fangs covered, since if you look at their skulls, their fangs don't go all the way past the jaw like Smilodon's.

And I think Thylacosmilus also had its fangs protected by skin when not in use, because why else would it have those ridiculous sheaths on its lower jaw?

Indeed, especially given that metatherians supposedly have restrictions on their lip anatomy, due to the joeys being attached to the teats. Such extravagant lip-interruptions in the lower jaw are unknown even in placentals.

As keeps cropping up in these comments, I'm not an expert on these many specific clades, but as far as I know, mekosuchines retain very crocodylid-like skulls, including all the ISO foramina etc. Given their nesting within a lipless clade and morphological similarity to living species, I'd probably stick with the lipless condition.

Fantastic article, as always. BTW something I find intriguing about your Inostrancevia is how much it (in my opinion at least) resembles some of the Crystal Palace sculptures. Did you draw any deliberate inspiration from them or is the resemblance purely coincidental?

This is fascinating stuff, thank you. I was reading about dinosaur lips in "The Tyrannosaur Chronicles" recently, but it's otherwise a new subject for me, and a very interesting one. As a special prize (!) here's a closed mouth, hidden teeth T. rex from my childhood Bible, "The Collins Book Of Dinosaurs" (1978), written by Tom MacGowen, and illustrated by Rod Ruth. This is, of course, prior to the toothy trope you mention above, which you date as from the 1980s and onwards. Still a nice little trend-busting sketch though!

Thanks for the thoughts, I wanna comment about this "All this said, what can we say about the decisions to show prehistoric animals with exposed teeth? My reading of modern tetrapods is that covered teeth is their ‘default’ configuration, and we should apply the same logic to extinct animals. If so, maybe only the more extreme examples of fossil dentition should qualify for perpetual display. Perhaps instead of asking ‘does this animal have lips?’ we should ask why they should not have them. We have to concede that the dentitions of many fossil animals frequently shown with exposed teeth – particularly theropod dinosaurs, gorgonopsids and other carnivorous stem-mammals – are relatively no larger, and in some cases a great deal smaller, than those enclosed inside the oral tissues of living animals, especially once taphonomic tooth slippage is corrected (above). For these species, it is very difficult to justify why their teeth should not be covered." Well, the reason you gave for why we should make theropods with covered teeth is far more logic than that "enamel needs moisture" thing, but that is not how it works. Well, crocodiles could have evolved with sensors only close to the nostrils, with about half of their teeth covered with lips (but they did not). Some birds have more oral tissue than other birds, etc. We can simply not be sure if a certain theropod had covered teeth unless extremely preserved oral tissue is found.

Dovahkiin HU3BR: I agree that we can't be sure until direct evidence is found, but this is the case for anything in science. Until then, the best we can do is make predictions using available data. An increasing number of authors have commented that living archosaurs are probably of limited use for inferring dinosaur oral tissues because they're so radically different to each other and specialised to flight or aquatic habits. This necessitates looking to other, more comparable tetrapods for info on dinosaur tissues, and a lippy covering is strongly indicated by them. Fully exposed teeth, by contrast, are much harder to justify given their rarity, and association with highly specialised forms, in the modern day.

I know, but we must notice that the great majority of reptiles are eighter venomous (but not like the way vipers are venomous) or at least have vestigial venom glands. My point? Vipers and few lizards like the gila monster do not require on moisture to put their venom into the prey's blood stream, but the mouth has to be moisty for this to work. Take a look http://www.livescience.com/498-surprising-origin-venom-revealed.html if this is right, the common venomous ancestor of modern squamates lived on an age where almost everything was quite big if compared to it, it would need to use venom to hunt. In the link it is also stated that "The common ancestor had venom glands on both its upper and lower jaws. Since then, snakes have evolved to having glands on just their upper jaw – glands on the lower would make it difficult to swallow prey", what is another clue of how it used it's venom. In the end all modern squamates have covered teeth mainly because their common ancestor needed moisture to use it's venom or for being extremely closely related with venomous squamates.

Mammalian lips are muscular and thus can help the animal in showing if it is angry or not, so a mammal would have more disvantages than a theropod if it had exposed teeth.

But since evolution hjas no goal it is completely possible that many, if not most, theropods had covered teeth. But thanks to their major evolutionary differences from mammals and squamates, I find it kinda difficult to believe that all theropods had covered teeth.

I'm not up on fish anatomy enough to comment specifically, I'm afraid. But I remember palaeoartist Bob Nicholls once pointing out how daft it is that fish are always restored as highly skeletal given how many modern species have soft faces, and I agree with this. Presumably (and shooting from the hip here - again, I'm not an expert) the cutting surfaces of Dunkleosteus jawbones had to be accessible and not buried under masses of tissue, but that doesn't mean a thicker skin could not cover the face right up to them (thinking of parrot fish and such).

Placoderm armours were at least partly internal due to nerve cannals and other features (as well as being basically the direct ancestor to the vertebrate skeleton), so lips are logical, or at least parrotfish-like more muscular skulls up to the blades

Fantastic post. Very informative. It's given me a bit of insight into a couple of things I've wondered about - those Smilodons and crocodylians.If you ever wanted to collect (and perhaps, add to?) these palaeoart-related articles in another book, you'd have at least one customer, here. I have to say I think your own palaeoart is getting better and better, too.

A very good read. I must ask though, is there any potential correlation between "lipless-ness" in crocodilians and aquatic ambush predators? Snapping turtles and moray eels also tend to have jaws that don't completely seal shut. And could this seemingly niche trend of retracted facial tissues present any parallels to pterosaurs (particularly pterosaurs suggested as "river-dolphin-like" in theorized behavior and dental anatomy such as Guidraco)?

Hi Mark thanks for link to my post. I do agree with the general premises and observations from this post mainly that 1) Covered teeth should be our default - the null hypothesis if you will and 2) the relationship between exposed enamel and the environment is complex.

However I can't help but feel that my arguments I made in my several posts have been either misconstrued, misinterpreted, or need to be reread. You say that my conclusions "rests heavily on enamel being a no-no in exposed teeth, I'm unconvinced for the reasons above" . However I did two posts on the subject and in the post that you linked to I was yet to come across Reisz & the whole exposed enamel argument. My main point then was that smilodon sabers with the amount of fleshy oral tissue generally depicted would not be able to make the precise cuts and tactile bites that modern felids do as the sensitive nerve pad and whisker area would quite literally be pushed back away from the bite zone. Anton himself points out the extra large infraorbital foramen in sabertooth predators and likelihood that it infers a highly sensitive nerve pad. Unfortunately he doesn't make the next logical step surmising that for such a nerve pad to work in conjunction with extra large fangs the nerve pad must be absolutely larger to be in contact with prey and allow a precise bite.Yeah I Said it... Every Sabretooth Image Every - OBSOLETE This more mechanistic approach to smilodon biting, in conjunction with the inferred superior oral protection deduced by mandibular flanges in other sabertooths, and the ubiquity of sheathed teeth in ALL terrestrial and aquatic carnivorans was ample evidence for me.

BTW I don't agree with the "hydration hypothesis". Water can strip enamel of its ions over time. It is saliva that protects enamel not water.

This is why crocs, delphinids have but a paltry layer of enamel on their tooth. Pinnipeds on the other hand which have thick enamel like other carnivorans maintain their enamel crystaline structure at optimal levels with a closed oral seal, big lips, and buffering salivary broth. It's not so much that enamel can't be exposed to the environment it's more about keeping that enamel at optimal health and strength.

None of the warthogs, fanged deer, elephants and other critter compare to smilodon in terms of relative investment in enamel nor do they absolutely depend on strong enamel to make a living. Smilodon absolutely needed that enamel to make a living, it had a definite cemento-enamel juncture, and it would have covered those pearly whites in a lubricating, protective, broth of saliva for peak performance. Just like every other carnivoran of the land or sea.

Thanks for the comments, Duane, and thanks for your patience with my response. Also, apologies if you feel I've mangled your argument. I appreciate your discussion about Smilodon teeth is multifaceted, but a one sentence summary of your idea is that enamel for 'functional' teeth needs to be kept healthy by staying inside the mouth, correct? If so, I don't think my description of 'this rests heavily on enamel being a no-no in exposed teeth' is too far off the mark, even if it is a very, very brief.

I must admit to not really agreeing with your arguments on machairodont teeth. Concerning their health, your argument relies on Smilodon having a unique canine function that requires extra care compared to living tusked species. While I agree that it's likely Smilodon canine function lacks exact modern functional analogues, the sabre teeth of tusked deer are not too far off in terms of physical demand (being routinely used in aggressive, tissue-tearing fights that leave victims scarred and wounded) and anatomy (thin layers of enamel over much of the tooth - Smilodon canines are not thickly enamelled). You have argued that the teeth of machairodonts are essential to the survival of individuals because of their role in predatory acts, and thus require extra care. Well, tusked deer canines are pretty important too: males use them frequently to see off intruders and thus protect their territory and resources, and, without them, they have no chance of siring offspring. While tusked deer aren't a perfect model for sabre-toothed predators, they suggest that exposed sabre-teeth can used in physically aggressive roles somewhat analogous to predatory biting without being maintained in chambers of invigorating saliva. I'm not sold on the idea that teeth used in an antagonistic manner require sheathing to retain good health.

On infraorbital foramina size: given the dual purpose of this opening (nerves and blood vessels) why are big lips any more likely than sensitive whiskers? Big infraorbital openings can mean different things in different species.

Finally, and echoing some other comments in this feed, your proposed model requires a level of soft-tissue lippage which not only opposes the feline EPB, but would be unprecedented among any living mammal. How would lips of that size actually function? I can't fathom how the obicularis oris would work in your model: this would be enormous, dropping ring of muscle around the mouth, and I'm not sure how it could be cleared from the teeth when biting and feeding, assuming normal principles of mammalian face musculature. It's possible that Smilodon had a unique facial myology of course, although the consistency of face muscles across mammals argues for this being unlikely. Muscles controlling the lip would need to be very large, and I expect we'd see some indication of this on the skull itself as we do in other mammals with sophisticated facial soft-tissues (I'm thinking trunks and proboscides here). Generations of people who know carnivoran anatomy very well have pored over machairodont skulls and never commented on such features. Maybe it's worth attempting to reconstruct the musculature of these animals to see what looks credible?

@Mark Witton as mentioned by another commenter, this highly detailed yet highly believable reconstruction by the artist known as Unlobogris seems rather fitting:http://unlobogris.deviantart.com/art/Where-are-the-fangs-here-there-are-611345739It's not "overly long" and easily sheathes the teeth when closed, and is still reminiscient in modern Carnivorans, albeit on a slightly larger scale. In the top right reconstruction, it looks almost identical to modern felids except the upper jaw droops down farther in the back to effectively cover the teeth.I know I'm not on an academic level as you or Duane, but my reasoning why larger lips are more likely than massive whiskers is because it's simply the default trait. Why grow massive, specialized whiskers when you can simply expand the lip that is the default function? As seen in modern Carnivorans and in their evolution of increasing canine size, as their teeth grow so does hte lip to accomodate it. It's simple. The same must have happened in Machairodonts, but once you get anything bigger than the canines of say, Homotherium, the lips expand larger to properly go along with this ever increasing tooth size. Why stop easily growing the default trait of lips to grow enormous, specialized whiskers not seen in any modern mammal (well, not as specialzied and enormous of this supposed Smilodon scenario that is)? After all, you said it yourself, the question is not why would it have lips, but rather why wouldn't it have lips.

The problem here is that explanations other than giant lips are not being explored. The base assumption is "big infraorbital foramina = big lips", but this is far from the only, or even most likely, explanation for big IFOs.

Firstly, this huge lip hypothesis predicts that there should be a link between canine size and IFO proportions in sabre-tooths. With the caveat that I'm not a cat expert, a brief scan of literature suggests this is not the case. The big toothed Xenosmilus has a smaller IFO than the smaller toothed Homotherium and Panthera, for example, while massive-toothed Smilodon has a particularly big IFO. That's only four taxa of course, but there's no obvious pattern there. Moreover, it's interesting that in all the assessments of cat skull morphology and metrics out there - and there are lots - that no-one has noted sabre-toothed species have IFOs which are consistently outliers to shorter-toothed species. Nor does there seem to be an obvious correlation between the extent of lip muscule scarring and canine size - and fossil cats are some of the best examples we have of fossil animal facial reconstruction. These are both problems for the lipped hypothesis.

Secondly, as others have argued, the functionality of IFOs means that their size may correlate with soft-tissues other than lips. There are two huge nerves which run through that opening, and having a sensitive set of tissues around the snout is not an unreasonable idea to explain an usually big IFO. Alternatively, sabre-tooths have slightly longer faces than other cats, so maybe they just need a bigger IFO because of slightly longer lips. Given that there does not seem to be a huge jump in size between the IFOs of regular-toothed cats and sabre-toothed ones, a slightly bigger IFO might be more likely to correlate with slightly enhancement of existing tissues, not a significantly more developed one (e.g. a massive set of dangling lips).

Thirdly, there's more to IFO variation than just size: attitude, outline shape and position on the skull are also variable. In some cats, like Asian lions, the IFO is even split in two. Moreover, IFO size changes with age in some species. These facts immediately ring alarm bells about comparing the raw size of these openings because things like the basic geometry of the skull - influenced by taxonomy and age - come into play. With the IFOs being differently placed, facing slightly different angles and being based on morphologically different skulls, their size is going to vary depending on how the pathway of the nerves and blood vessels intersect their respective bone walls. Basic skull geometry may account for much of the difference we're ascribing to massive soft-tissue development.

All this said, there're lots of factors unrelated to soft-tissues and lip size to consider here, and a giant set of lips can only be considered parsimonious once the influence of these other factors have been excluded. As I've said, I'm not a cat expert, but it seems the burden of proof lies with proponents of the lipped hypothesis, not the other way around.

"As seen in modern Carnivorans and in their evolution of increasing canine size, as their teeth grow so does hte lip to accomodate it."

Can you cite an example of this? I'm not an expert on this group, but I didn't see any evidence of lip/tooth size correlation in this group when researching this piece. They all seem to have the same 'overlip' configuration described in the article.

"Why stop easily growing the default trait of lips to grow enormous, specialized whiskers not seen in any modern mammal (well, not as specialzied and enormous of this supposed Smilodon scenario that is)?"

But you're arguing that these animals would grow lips unlike virtually all modern cats, and unlike most other carnivorans. These animals let their canines overgrow their lower lips without worrying about pouches to accommodate them. For Smilodon to have a lower lip that drooped to meet a giant, overhanging upper lip it would need a tissue configuration akin to the derived condition of one, unusual living cat, and not like the vast majority of its living relatives. Moreover, as mentioned several times above, there are scars that anatomists and artists use assess muscle extent in fossil cats, and those of Smilodon do not seem unusually big or deep.

"you said it yourself, the question is not why would it have lips, but rather why wouldn't it have lips."

The fact that no modern animal with equivalently sized teeth sheaths them has to be a clue, right? Smilodon has proportionally larger teeth than any tusked deer and is more akin to a walrus: all these species have exposed teeth, so why not this cat? As outlined in the comments and article above, there is no functional or histological reason to think their teeth could not be exposed.

I see you only commented on my 1997 Prehistoric times article, not the actual peer reviewed article of the same year (which is now available on Academia), or my 2015 PT article, or my poster I presented at last years SVP (also available on Academia). I'll just say, we agree to disagree.

Thanks for the comment Tracy, and for making the peer reviewed paper available - I was unaware of it. I hope it's clear that this article is not meant to be a review: there are lots of other works I could have cited, but I only chose a few examples.

Regarding the 1997 paper: It seems the arguments are essentially the same as the PT article concerning jaw closure, tooth overlap and so on however (although obviously more detailed and technical), so I think my comments above above about 'calibrating' a degree of acceptable dental overhang with modern animals, the assumption that osteological data alone can largely inform this discussion etc. are still relevant.

Also, I have a question about using the 'Stan' skull in this sort of assessment, seeing as - for whatever reason - it seems to have a much larger overbite than other Tyrannosaurus. The AMNH type skull, 'Sue', 'Jane', 'Samson' and so on seem to have much smaller overbites, more typical of those of other theropods. It also seems that others - Hartman, Paul, etc. - restore Stan with a smaller overbite than you figured, as well as much shorter (i.e. resocketed) teeth. These points raised, it seems 'Stan' is a somewhat aberrant skull and open to several interpretations. If so, is it the best skull to use for modelling lip tissues, when there are other, more readily interpreted/less controversy-prone skulls available? What would your verdict be like with other Tyrannosaurus, or other theropod skulls?

I've been wondering if Stan's extreme overbite might be partially due to some distortion during the fossilization process. MOR 008 also seemed initially to have a big overbite, but as this GIF demonstrates, if corrected for possible distortion, it's not as extreme.https://1kvzgw.bn1.livefilestore.com/y1pZuAhVuYWYM2YxliLrpNUW9mZWHvG39B2LchKtp8ooOVdbeTd6INTZDpNXTmIxyCzo2M1Jv8Jf898xZ_haCA0DnkprnydhGRg/mor008.gif

Not to beat a dead horse but "functional teeth need to be kept inside the mouth" is not how I would describe my argument.

Again I agree with you that there are many shades of grey here and many - at this point somewhat subjective - arguments to be leveled. Pterosaurs, plesiosaurs, crocs all kept enamel enshrouded teeth exposed to the environment. So too do tusked mammals to varying extents keep enamel covered teeth exposed to the environment.

But in the case of crocs, pterosaurs, and plesiosaurs loss of enamel by chemical, abrasive, or other means is not a big problem as they simply grow more teeth. In tusked mammals the slight shellac of enamel or band of enamel may or may not last the lifespan of the animal but again, no big problem, because dentine and cementum grow continuously.

Enamel can not regrow but it can be maintained via saliva which is supersaturated in calcium and phosphorous. There is a rich literature on this of course but it is not found in paleontological sources but in the dental literature which I would encourage inquiring minds to dive into.

Smilodon does not fit into either of these two general groups. It can not grow new teeth; it has enamel covered canines not covered in cementum or dentine like ever-growing tusked animals. All of these animals put their teeth through rigorous abuse but only smildon is left with no protective benefits for it's teeth? Can't grow another tooth like a croc or pterosaur; can't generate more dentine like an elephant or walrus; no chemical buffering via saliva either?

Mark you say "smilodon canines are not thickly enamelled" in your comparing their canines to the slight shellac of enamel on tusked deer. I do have to request some sort of reference for this. Everything I have come across shows a definite cemento-enamel junction and a typical crown of enamel like all other carnivorans.

With enamel I believe it is a question of investment. Why invest a solid crown of highly mineralized, expensive to produce enamel exposed to the outside environment if you can instead just put on thin shellac of enamel and/or let dentine do the work. Dentine is softer, cheaper, can be produced non-stop, and is just good enough to do the trick. If Smilodon was shown to display the characteristics of tusked mammals in this regard I would stand down. It has not though and as far as I have seen its histology matches most closely to ever other carnivoran.

Good point on obicularis oris. The reason no researchers have found anything to suggest such a morphology is that perhaps there is no osteological correlate to give such a signal? It is bound to other muscles but is itself just sort of free floating. One thing mammals can do very well is funky stuff with their facial muscles and I don't see why a cat could not as well.

And here is an interesting test with regards to the hydration hypothesis: investigate pinniped teeth versus toothed whale teeth. Pinniped have kept enamel crowned teeth - not coincidentally they also cover their teeth - even underwater - with lips and buffering saliva.

http://link.springer.com/article/10.1007/s00114-016-1366-z

Toothed whales on the other hand have abandoned lips, have no or vestigial salivary glands, and have de-invested in enamel.

Again - shades of grey - but I see some trends starting to emerge. Lips, saliva, tooth regeneration or lack-there-of, enamel investment, ever -growing dentine, lifespan of a tooth. All of thee factors play a role. Hydration of a tooth? Maybe not so much.

"In tusked mammals the slight shellac of enamel or band of enamel may or may not last the lifespan of the animal but again, no big problem, because dentine and cementum grow continuously."

Well, it seems to in some species! Look at the deer and pig teeth illustrated above, and check out some of the references mentioned in the text. Their enamel is not worn off like it is in elephants or walruses, and it covers half or more of the tooth. It may be that there's a behavioural reason for this - they don't abrade their teeth like elephants or walrsuses do. However it happens, it seems some mammals are capable of bearing exposed enamel.

If that is indeed the case that Smilodon actually had a thin layer of enamel that changes my stance on things a bit. I was under the impression it had full on crowns of enamel but if it only had a thin veneer this fits more of the pattern I have been noticing for exposed enamel species.

I am grateful that we can have this discussion without charges of " that can't happen because it looks ridiculous". Smilodon I know was always a tough sell for covered lips but I do think in shifting the goal posts a bit people have opened their minds to other less extreme sabertoothed predators being a bit more covered than generally restored. I still suspect that for the other reasons I have listed Smilodon did not simply look like a lion with giant fangs spliced onto it. I think it had a heavier muzzle, thicker gingiva, and nerve pad for fang protection and sensing prey movement while engaged with it. And a bit more elastic lip to allow for that extreme gape. But I relent, sabers probably were partially exposed.

@Chris Srnka I don't know. MOR 008's skull is pretty complete. I doubt that much distorsment would happen to the skull. It's just that the lower jaws weren't full preserved (that would account to why most depictions show an overbite).You can still get a skull length of 1.55 meters and still have no overbite for MOR 008.https://cdn.discordapp.com/attachments/236268358146719754/238869744969711617/Screen_Shot_2016-10-20_at_8.42.44_PM.pnghttp://paleojoe.deviantart.com/art/Revised-MOR-008-skull-Tyrannosaurus-rex-471746112

I don't have anywhere near the right kind of education for this, so you can take my highly scientific gut feeling with as much salt as you like; but the sheer extent of soft tissue needed to cover Smilodon canines, at least, doesn't sit right with me. With the other 'sabre-toothed' examples - monitors, mandrills, hippos, clouded leopards, gorgonopsids, tyrannosaurs - the canines don't go too far beyond the contours of the skull, if at all. (Although Inostrancevia's jaw has a little leg-up in that regard) You need to puff the lips out a bit, form a couple of sockets, bingo.Looking at Smilodon skulls and reconstructions, just over half the tooth's length dangles below the mandible, about 8-9cm from what measurements I can find online. It means a heck of a lot of soft, fleshy chin to house a pair of sockets. Is it reasonable? I don't know. Gorgonopsids and Thylacosmilus went further with bony structures, and even water deer Hydropotes inermis can do a mean Bruce Forsyth/Jay Leno impression. But is full envelopment necessary? Going by something I've recently read...

I'll side with Warren here. The teeth of things like Neofelis do project a little beyond the mandible, but they're within a sensible margin for sheathing. Indeed, they aren't any more extreme than what we see in other, fully sheathed mammals. But, as Warren says, Smilodon is a different kettle of teeth altogether, and would require an unprecedented amount of soft-tissue to cover its canines. As mentioned above, there are unanswered questions about how such a facial musculature would work because the contour of the lips would be radically different to anything alive today. The 'unsheathed' model of tradition wouldn't have these problems, though, and might be more parsimonious on at least these grounds.

Very good post, both in the blog itself and the comment section! This a huge subject of interest to me as a paleoartist. But I must ask, What are your opinions on Witmer and colleagues studies on lips/cheeks in Dinosaurs?

They seem to restore dinosaurs with at least lips now - the 90s 'fully beaked' ceratopsian idea seems to have fallen away. I know at least Casey Holliday has no problem with expanded rictal tissues in some species, and I agree.

May I just point out that Uintatherium is generally restored almost exactly like your baboon and hippo? It's not an animal I generally pay much attention to, but it'd be pretty interesting to see that head in a modern restoration.

I really love reading these articles, it's so nice to come across someone who actually does their unbiased research regardless of opinion or what "looks cool". Personally, the JP T. Rex looks like my grandpa with his ever-angry eyebrows and buck teeth.

(And thank you for giving me hope that Smilodon's face didn't look like a crippled bulldog.)

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Nice blog. This is a very good blog on Dinosaur fossil teeth. I would like to thank you for all the information you give. Its really important to get the information on dinosaur fossil teeth. So thenks for the information you give.

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Who is this 'Mark Witton' chap?

Dr Mark Witton is a palaeontologist and palaeoartist, affiliated with the University of Portsmouth, UK. My technical research is focused on pterosaurs - Mesozoic flying reptiles - but my artwork has introduced me to a wide array of different fossil animals that are just as interesting. I work as a freelance author, consultant and artist: check out my work at MarkWitton.com, follow me on Twitter @MarkWitton, and browse my books here. Contact me at wittonprints[at]gmail.com. Due to volume of email I can't always reply to messages, but I do my best.